<template>
  <div id="goTop">
    <div class="goTop" v-show="goTopShow" @click="goTop"><i class="goTopIcon"></i></div>
  </div>
</template>
<script>
export default {
  name: 'goTop',
  data () {
    return {
      scrollTop: '',
      goTopShow: false
    }
  },
  methods: {
    handleScroll () {
      this.scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
      if (this.scrollTop > 500) {
        this.goTopShow = true
      }
    },
    goTop () {
      let timer = null, _that = this
      cancelAnimationFrame(timer)
      timer = requestAnimationFrame(function fn () {
        if (_that.scrollTop > 0) {
          _that.scrollTop -= 50
          document.body.scrollTop = document.documentElement.scrollTop = _that.scrollTop
          timer = requestAnimationFrame(fn)
        } else {
          cancelAnimationFrame(timer)
          _that.goTopShow = false
        }
      })
    }
  },
  mounted () {
    window.addEventListener('scroll', this.handleScroll)
  },
  destroyed () {
    window.removeEventListener('scroll', this.handleScroll)
  }
}
</script>

<style scoped>
  .goTop{
    position: fixed;
    right: 10px;
    bottom: 50px;
    width: 30px;
    height: 30px;
    background:rgba(0,0,0,.35);
  }
  .goTop:hover{
    background:rgba(0,0,0,.35);
  }
  .goTopIcon{
    display: block;
    width: 30px;
    height: 30px;
    background-image: url("");
    background-repeat: no-repeat;
    background-position: center center;
  }
</style>
